Contacts on switching devices, for example electronic relays, can weld together when subjected to high-current resistive or inductive loads. The weld occurs due to the transfer of contact material when the switching device contacts are separating or closing. An electrical arc occurs naturally between the switching device contacts immediately upon separation of the contacts for resistive loads and upon closing of the contacts for inductive loads. This arcing causes material to transfer from one contact to another. Over time, this arcing can cause the switching device contacts to become pitted and eventually weld together. Welding together of the contacts can cause unintended results, such as causing an attached circuit to be unintentionally energized.
To prevent such arcing and welding, some controls are designed with switching devices located on both sides of the incoming load (i.e., on both the power side and the neutral side) instead of just on the power side. By providing this redundancy, a single welded switching device will not cause the circuit to be energized inadvertently. Under this scenario, a single welded contact on a switching device is not detectable since the circuit will continue to function correctly. Furthermore, it requires that the contacts of both switching devices be welded before indicating a failure. There is therefore a need for a method for sensing when a contact on a switching device has welded together in circuits having switching devices on both sides of an incoming load.